MEXICO CITY — An indigenous activist in Honduras who won a prestigious international environmental prize for fighting a dam project despite continued threats was assassinated on Thursday in her hometown, officials said.

Gunmen broke down the door of the house where the activist, Berta Cáceres, was staying in La Esperanza, in western Honduras, and shot her early Thursday, human rights groups said.

Ms. Cáceres, 44, had led a decade-long fight against a project to build the Agua Zarca Dam along the Gualcarque River, which is sacred to the Lenca people. The campaign involved filing legal complaints against the project, organizing community meetings and bringing the case to the Inter-American Human Rights Commission. Last year, she won the Goldman Environmental Prize, which is awarded to grass-roots leaders who build community support to protect the environment.

The killing of one of Honduras’s most prominent environmental activists casts attention on the country’s dismal human rights record. It comes just after President Juan Orlando Hernández traveled to Washington and Mexico City last week to argue that his government was turning the corner in combating the violence that makes Honduras one of the most murder-plagued countries in the world.

Although the Inter-American Human Rights Commission had ordered protective measures for Ms. Cáceres, she was not under the protection of the Honduran security forces on the day of her death, Julián Pacheco Tinoco, the Honduran security minister, said at a news conference in Tegucigalpa, the capital. He said she was not in the place she had reported as her home when she was killed.

Her brother Gustavo Cáceres told media outlets in La Esperanza that her death could have been avoided. “The police were responsible for providing security for my sister here in the city,” he said. “She wasn’t hiding.”

The killing brought widespread condemnation from international human rights organizations and American lawmakers. Mr. Hernández, the president, called the killing of Ms. Cáceres “a crime against Honduras, a blow for the people of Honduras,” and promised an investigation with help from the United States.

There are no immediate suspects, officials said. Conflicts between local communities and large companies in Honduras often draw in several armed groups, including the army.

“Everyone is saying that the government or the company did it, but you’ll never know,” said Ms. Cáceres’s nephew, Silvio Carrillo, in a telephone interview from Oakland, Calif., where he lives. “It’s the art of obfuscation.”

Since 2013, Ms. Cáceres’s organization, the National Council of Popular and Indigenous Organizations of Honduras, has protested to try to stop the dam’s construction. Under international law, indigenous groups must be consulted on projects that affect their lands, but the Lenca say they were not consulted about the dam. They maintain that the 22MW hydroelectric project, which would create a 300-meter long reservoir and divert 3 kilometers of the river, will jeopardize their water resources and their livelihood.

For over a year, the organization maintained a blockade to prevent access to the site despite attempts by security officials to evict protesters. In July 2013, a Honduran soldier fatally shot Tomás García, another leader of Ms. Cáceres’s organization, during a peaceful protest.

The protest prompted the Chinese company Sinohydro, which had the contract to build the dam, to withdraw from the project.

The Honduran company behind the dam, Desarrollos Energéticos S.A., continued with the project, however, and Honduran business leaders took up the cause against Ms. Cáceres.

Aline Flores, president of the Honduran Council for Private Business, said in 2013 that the groups led by Ms. Cáceres were “boycotting, invading and making Honduras look bad internationally.” Criminal charges were filed against Ms. Cáceres, first for carrying an unlicensed weapon, which she said had been planted by military officers at a roadblock, and then for incitement.

The company resumed construction of the dam last fall, avoiding potential blockades by moving to the other side of the river, Ms. Cáceres said at the time.

Over the past month, the threats against Ms. Cáceres and her organization had mounted after security forces detained more than 100 people during a peaceful protest on Feb. 20.

Since a 2009 coup in Honduras, journalists, judges, labor leaders, human rights defenders and environmental activists have been assassinated in targeted killings, with their murders often going unsolved. Twelve environmental defenders were killed in Honduras in 2014, according to research by Global Witness, which makes it the most dangerous country in the world, relative to its size, for activists protecting forests and rivers.

In accepting the Goldman prize, Ms. Cáceres described what it had been like to live under siege.“They follow me and threaten to kidnap and kill me,” she said. “They threaten my family. This is what we have to face.”

A version of this article appears in print on , on Page A9 of the New York edition with the headline: Indigenous Activist in Honduras Is Killed. Order Reprints | Today’s Paper | Subscribe